Tree-ring reconstructed hydroclimate of the Upper Klamath basin

•The first tree-ring reconstructions of the Klamath River basin hydroclimate.•Longest reconstruction reaches 1000 CE.•Short and long dry periods were likely more severe from the 11th-to-13th century.•The early 20th century likely contains the most severe mid-length dry periods.

SummaryThis work presents the first tree-ring reconstructions of hydroclimate for the Upper Klamath River basin, which stretches from northern California into southern Oregon. The extended record provides a centuries-long perspective on the region's hydroclimatic variability and context for water-related political issues that have erupted in recent years. Reconstructions of water-year precipitation for Klamath Falls, Oregon (extending 1564-2004 and 1000-2010 CE) were developed to compare past drought severity with drought severity of the instrumental record (extending 1896-2011). The reconstructions suggest that variability exhibited during the instrumental period captures extremes of moderate-to-long-duration (6-, 10-, and 20-year) droughts, but not of short (single-year and 3-year) and very long (50-year) droughts, which were more severe during the 11th-13th centuries. The late-16th-century “mega drought” is present in the Klamath River basin, though with less strength than in the neighboring Sacramento River basin. Cool-season storm tracks appear to be a direct driver of hydroclimatic variability, leading to instances of see-saw like relationships with neighboring regions, such as in the mid-14th century. In contrast, the larger area of drought in the 12th century is suggestive of a long-term northward shift in cool-season storm tracks.